The production of integrated circuits requires the use of semiconductor wafers, also known as substrates. Processing semiconductor wafers requires multiple steps. At each step, layers of material are deposited and processed on the wafer surface. The wafers are typically circular, generally 0.031 inch thick and in the range of 100-300 mm in diameter. Accordingly, the wafers are very fragile.
The deposition the materials upon these fragile substrates requires that the wafers be coated with numerous solutions and then rinsed. It is important that the wafer remain extremely clean and be completely dry before the next processing step begins. To remove the chemicals from the previous step, the wafers undergo a rinse and dry cycle between processing steps. Generally, deionized water rinses impurities or contaminates from the wafer, after which a separate process dries the wafer.
Various processing devices perform the task of rinsing and drying. U.S. Pat. Nos. 4,300,581, to Thompson and 4,571,850 to Hunt, et al. describe one such device in which a wafer carrier connects to a frame that rotates while stationary nozzles spray fluid to rinse the wafers, or alternately, inject heated gas to dry the wafers. A carrier is a device which holds a number of wafers during the processing steps and during transportation from one step to another. The rotation causes water or moisture to be centrifugally thrown from the wafer and carrier.
Although centrifugal-type dryers provide distinct advantages over the prior art, certain disadvantages still exist. For example, one disadvantage of centrifugal drying is that water marks may form on the wafer. Water marks result when some of the water dries on the wafer instead of being thrown off during rotation. The residual water, along with the spinning of the wafer, may leave streaks which run from the center of the wafer to the outside edge, i.e., water marks. Water marks damage the surface of the silicon wafer and are therefore undesirable. Therefore, a need still exists for a drying method and apparatus which minimize such marks.
In response to this need for a drying method which does not leave water marks, a system was developed which slowly lifts the wafers out of a deionized water bath while exposing the wafer surface above the water line to a solvent vapor. The resulting wafer is entirely void of water because, the vapor, which plays an essential role in this drying process, interacts chemically with the water to decrease the water's surface tension and adhesive strength. More simply, the vapor absorbs into the top edge of the water bath and inhibits the formation of water droplets which would otherwise form on the wafer. However, for this method to successfully inhibit the formation of water droplets, the wafer must exit the water very slowly. Thus, for a normal size wafer, the time to complete the drying process commonly exceeds ten minutes.
U.S. Pat. No. 5,143,103 issued to Basso, et al., U.S. Pat. No. 5,054,210 issued to Schumacher, et al., and U.S. Pat. No. 4,841,645 issued to Bettcher, et al. adopt this method of drying. The preferred solvent is isopropyl alcohol (IPA), however, other vapor from other solvents achieve a similar result, i.e., reducing the surface tension of water. Drying systems utilizing this method have the advantage of drying silicon wafers with low contaminant particle counts while leaving the wafer surface free of water marks.
However, a common disadvantage to these substrate drying methods is the length of time required to perform a dry cycle. For example, a normal spin dry cycle takes 5-6 minutes. The average cycle length for the method which lifts the wafers out of water and into IPA vapor generally exceeds 10 minutes. Dry cycles of this length slow substrate production.
An alternative method for drying wafers combines spinning and IPA liquid. Japanese Patent 402237029 discloses spinning a wafer while dripping or pouring liquid IPA onto the center of the wafer. The liquid IPA, in conjunction with the water or moisture, is thrown from the wafer from the rotation. This method, although preventing water marks, has serious drawbacks. First, the liquid IPA is reactive, i.e., it can corrosively damage the fragile surface of the wafer. Second, this method uses relatively large amounts of IPA, which is undesirable because of government regulations regarding the amount of solvent which may be released into the environments. Thus, this method may require additional apparatus to limit IPA release into the environment. Third, it is difficult to dry multiple wafers simultaneously using this method because it is difficult to drip or pour IPA liquid onto multiple spinning wafers. IPA is also flammable; and using this method, which has high IPA concentrations, may increase the risk of fire or explosion.
The background section of U.S. Pat. No. 5,050,126 to Moe makes reference to the use of IPA with a centrifugal dryer, but no details of such an arrangement are given. It is also understood that IPA liquid has been sprayed into a centrifugal dryer. However, this is not vapor and a relatively large amount of IPA would have to be used. Furthermore, spraying IPA into the dryer does not provide a uniform application, which leads to ineffective drying.
U.S. Pat. No. 5,271,774 discloses a single wafer horizontally positioned on a centrifuge and a bubbler for conducting a carrier gas through an IPA reservoir and into a chamber surrounding the centrifuge to facilitate removal of a film of liquid on the wafer. The liquid combined with the bubbler gas is directed out of the chamber.
Notwithstanding these prior efforts, a need exists for a silicon wafer dryer which (1) drys a cassette load of wafers quickly, (2) minimizes the presence of water marks, (3) reduces particulate contamination, (4) uses relatively low amounts of alcohol or solvent, and (5) decreases the risk of fire or explosion.